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That means that the height should be the same as the height of it's container, in this case, #left_col or #right_col.

Do you want the iframes to be 100% of the window size? If so, you'd need to make every parent element 100% in height also, up to the <body> element. After that, you might have some problems with (browser default) padding and margins; so you might have to change those to 0.

This is exactly what I'm trying to achieve (except for the fact that the middle section is another iframe, and all the three iframes can scroll if content height is >100%) but can't.
Please check here, for my thread on this: